How do you decide the sample size when polling a large population?

Australia is currently having an election and understandably the media reports new political poll results daily. In a country of 22 million what percentage of the population would need to be sampled to get a statistically valid result?

Is it possible that using too large a sample could affect the results, or does statistical validity monotonically increase with sample size?

Answer

Sample size doesn’t much depend on the population size, which is counter-intuitive to many.

Most polling companies use 400 or 1000 people in their samples.

There is a reason for this:

A sample size of 400 will give you a confidence interval of +/-5% 19 times out of 20 (95%)

A sample size of 1000 will give you a confidence interval of +/-3% 19 times out of 20 (95%)

When you are measuring a proportion near 50% anyways.

This calculator isn’t bad:

http://www.raosoft.com/samplesize.html

Attribution
Source : Link , Question Author : brotchie , Answer Author : Neil McGuigan

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